In a wireless communication system, "multipath" exists when there are multiple radio frequency transmission paths between the transmitter and receiver. If the secondary transmission paths can be characterized by the summation of several delayed and attenuated replicas of the desired signal, the multipath is called "specular multipath." This type of multipath may occur when the receiving antenna receives reflections from such obstacles as buildings, the earth, or the ionosphere. Receiving such multipath signals at the receiver may result in fluctuation of the received signal level. This is because the multipath wave is delayed by some time, .tau., compared with the direct wave. When multiple signals propagating over different paths combine at a subscriber or receiver location, fading may occur if the signals combine destructively.
Fading caused by destructive combination of multipath signals is frequency dependent. For example, if a signal from point A, the transmitter, takes multiple paths to point B, a subscriber, and these multiple signals having a first frequency combine destructively, a signal having a second frequency and traveling over the same multiple paths may combine in an additive manner at the subscriber location because of the difference in frequency. This is the well-known frequency selective fading phenomena.
In cellular communications systems, the use of adaptive antenna arrays has been proposed to improve uplink and downlink signal quality or to increase the range or capacity of cellular communication systems. Capacity may be increased because a subscriber's signal is transmitted in a direction so that the intended subscriber receives the signal while other subscribers may be spared from the noise of a signal not intended for them. Additionally, range may be increased because of antenna gain provided by a directional antenna.
An adaptive antenna array is composed of multiple spaced-apart antenna elements which may be driven by radio frequency signals having specific amplitude and phase relationships in order to control arid direct the antenna radiation pattern of the array. An adaptive antenna array may be implemented with a uniform linear array or other physical configurations.
Because fading characteristics are different at different frequencies, using an adaptive antenna array to form a downlink beam in the same direction as a most powerful uplink signal may not provide the best signal quality at the subscriber's location. This problem may occur in a communications system using an adaptive antenna array to direct antenna patterns toward a particular subscriber, the uplink signal strength received at the transmitter may have adequate signal strength while a downlink signal directed in the same direction as the uplink signal at the base station may not be received at the subscriber with adequate signal strength due to the difference in fading characteristics between the receive frequency and the transmit frequency. This is because signals on the uplink may combine additively while signals at a different frequency on the corresponding downlink may destructively combine. One known method of optimizing the efficiency of the traffic channel involves changing or perturbing the pattern of the traffic channel in order to find a better radiation pattern, or better radiation direction, for the traffic channel. The problem with this method is that the traffic channel may be degraded while the process searches for an improved radiation pattern.
Therefore, a need exists for an improved method and system of optimizing a traffic channel in a wireless communication system using adaptive antenna arrays and beamformed traffic channels between transceivers.